Merge pull request #202 from bakul/mk-c-api

Added a docment on the API. Fixed a few errors. A few more tests
back out "enable Assign to a slice" - #198
add test for d[!d] as Lvalue
re-enable "Assign to a slice" - #198
Fix kap(), kapn() and kapn_().
Added an example to use kap() with a dictionary, where keys
are column names and values column data. Added code to add
rows.