Previously the checker passed only the current buffer as input to the
kotlin compiler, everything else was passed as classpath. This meant
an access to an internal program element (function, type, variable, etc)
was reported as error. This commit adds a sourcepath option that will be
passed to the compiler during a check.
The syntastic checker uses a name for the classpath variable that is
different from what is produced by the gradle plugin. This resulted in
the classpath not being read from the generated configuration file (from
gradle)
