html table code of DataFrame and Vector are modified

[Shekharrajak]

It will be better if we use thead and tbody tag in the html table code of the DataFrame and Vector . Also I have defined to_html_thead and to_html_tbody method to get the header and body part of the html table. These things will be helpful for daru-view table ( generate table using DataTables gem ).

See how I am using this PR branch in datatables example to generate table code with pagination, search and other features. . Actually DataTables gem will check the thead and tbody of the table.

[v0dro]

@athityakumar use this in daru-io.

[Shekharrajak]

Few things that can be useful in future (TODO):

• df.to_html(options) : where options may contains table id, class or other styling to include in table tag. (this can be done using content_tag)

• In each row , first column must have th tag (first column is always index. Right now it is in tr tag. ) Just need to modify the tbody erb file.