Rename private functions

Adept common code convention and use double dash in private functions
names.
Rename company-ycmd-backend -> company-ycmd

Match common backend naming convention like other company backends.
Add support for inserting function arguments as a template after completion
Change annotations order
Add docstring to `company-ycmd--params' function
Merge pull request #47 from ptrv/company-ycmd-enhancements

Company ycmd enhancements