Add functionality for dynamic includes #93

As per the issue raise #93 and with some amendments to corrects the suggest fix. Variables can now be used with includes so that the contents of the variable is evaluated to a string and then the include statement is evaluated as normal with the value of the string as the file name (without extension or path) to search for.
Add functionality for dynamic includes #93

See previous commit for extended explanation.
Added test for dynamic includes
Updated dynamic includes resolution to read better.

Replaced path with templatePath as that is more semmantic and
lifted the templatePath definition out of the if statement so as to
prvent future jshint errors.
Merge branch 'master' of github.com:jasper-lyons/ejs
This is the initial fork of ejs with i (dynamic includes).
Docs update.