ENH: fast reads on large gzip files

Fixes #209
BUG: fix previous commit

Acutally use the opening logic
RF: put gzip chunk constant into module global

Allows user to set max chunk size.
Merge pull request #1 from matthew-brett/module-global-gzip

MRG: add module global to gzip as discussed
Merge pull request #210 from GaelVaroquaux/fast_gzip_read

MRG: fast reads on large gzip files

Fixes #209
FIX: Cleaner tests
FIX: Minor fixes
ENH: Add suppress_warnings
Merge pull request #254 from Eric89GXL/clean-tests

MRG: Cleaner tests

In the process of working on #253 I was a little bit surprised by the number of warnings during testing. This cleans that up, and fixes a couple minor errors along the way.
FIX: Support XYTZ order and multiple echos in PAR/REC files
ENH: Support for partial recordings, and overwrite checking
WIP: Add bvals/bvecs
FIX: Working for variable scale factors
ENH: Add dwell time
FIX: Verbose dwell output
FIX: Fix dwell time
FIX: Fix numpy
FIX: Move most work to parrec
FIX: Fix for truncated data
FIX: Fix tests