Added code to support non-standard cores, such as attiny.
Note that the semicolon notation is not yet supported. For example, using arduino:arduino
as a core won't compile.
Support use of colon (:) when specifying core and variant.
When using standard as well as non-standard cores and variants you can use : to specify the folder where
the core files or the variant exists. For example arduino:arduino corresponds to the arduino core found in the
path/to/hardware/arduino/ directory.
Support no-variant requests.
Some cores do not specify any variant in the boards.txt file, so the codedender site will send no variant field in the
build configuration. With these changes, this case is properly handled.
Do not add the path to the variants folder when variant is empty.
Merge pull request #4 from codebendercc/external_core_support

External core support