Feature table enhancements

[yudytskiy]

Hi.
I've made some changes to Table and _Cell proxy classes as well as oxml classes templates.
This patch adds borders property to the Table and _Cell classes that could be used to get access to borders, add and remove borders of table and table cell.
In addition table _Row class has dont_split property that implements CT_TrPr w:cantSplit tag
I have not added any tests to the project, sorry for that.

Definitely my code might be optimized and refactored, but I have tried to keep along with current code style and base structure.

Code examples:
Table borders

from docx import Document
doc = Document()
tab = doc.add_table(rows=0, cols=2)
tab.autofit = True
for side in ('top', 'bottom', 'start', 'end'):
    tab.borders.add_border(side, 'single')
tab.borders.add_border('insideV', 'double')
# Change line style for the top border.
# One can change folowing properties of border instance:
# name = 'top' | 'bottom' | 'start' | 'end' |  'insideV' | 'insideH'
# causes removing existing border with old name and creating copy of the border with new name
# line: 'single' | 'double' and many other types of line documented in <xsd:simpleType name="ST_Shd"> 
# size: unsigned decimal ST_EighthPointMeasure
# space: unsigned decimal  ST_PointMeasure
# color:  'auto' and others corresponding to ST_HexColor
tab.borders[0].line = 'double'  
row = tab.add_row()
# Table row now has dont_split property to avoid splitting paragraphs and runs of a row when page break accurs
row.dont_split = True
row.cells[0].text = 'Cell1'
doc.save('test_border.docx')

Table cell borders

from docx import Document
doc = Document()
tab = doc.add_table(rows=0, cols=2)
tab.autofit = True
row = tab.add_row()
cell = row.cells[0]
cell.text = 'Cell1'
for side in ('top', 'bottom', 'start', 'end'):
    cell.borders.add_border(side, 'single')
cell.borders.add_border('tl2br',  'double')
# One can change folowing properties of border instance:
# name = 'top' | 'start' | 'left' | 'bottom' | 'end' | 'right' | 'insideH' | 'insideV' | 'tl2br' | 'tr2bl'
cell.borders[0].line = 'double' 
doc.save('test_border.docx')