Create store layout if not present

[rahulmutt]

When you switch the store-dir to a non-standard location, etlas for some reason does not create the appropriate layout for the package db like it should.

etlas.exe: Failed to build eta-java-interop-0.1.3.0 (which is required by
text-1.2.2.2). The failure occurred during the final install step. The
exception was:
c:\st\eta-0.7.2.1\package.db\: openBinaryTempFileWithDefaultPermissions: does
not exist (No such file or directory)
Failed to build stringsearch-0.3.6.6. The failure occurred during the final
install step. The exception was:
c:\st\eta-0.7.2.1\package.db\: openBinaryTempFileWithDefaultPermissions: does
not exist (No such file or directory)
Failed to build transformers-0.5.5.0 (which is required by mtl-2.2.2). The
failure occurred during the final install step. The exception was:
c:\st\eta-0.7.2.1\package.db\: openBinaryTempFileWithDefaultPermissions: does
not exist (No such file or directory)

Found by @hmemcpy on Windows.

[jneira]

Closed by #94