Fix local fs support

[simleo]

This PR contains multiple fixes for the hadoop-on-local-fs scenario, i.e., the one where fs.defaultFS is set to file:///, and adds a new Travis job to test it.

Main highlights:

• Hadoop's pipes submitter is still broken due to MAPREDUCE-4000. However, this PR applies the required fix to our own submitter, so Pydoop now supports running mapreduce applications on the local fs via pydoop submit.
• The above issue further stresses the fact that the "official" way to run applications is via pydoop submit. Examples still relying on PipesRunner have been rewritten to use pydoop submit, so that they can run correctly on the local fs. Running via mapred pipes is still supported when it works, and we're checking that in int_test/mapred_submitter (which serves as a sanity check for our submitter).
• PipesRunner and related stuff can now be dropped, since they would just be a manteinance burden.