NF: 1st-level HRF-modeling for eventrelated_dataset() (fixes gh-188)

Quite simplistic, based in NiPy. So far without contrasts, but should be
easy to add.
Enable NiPy for travis
ENH: Custom regressors for HRF modeling (from dataset or external sources)
DOC: update and extend eventrelated_dataset() documentation.
BF: Guard against missing externals
DOC: minor typos/wording for glm/hrf new functionality docs
RF: Move NiPy's GLM into its on mapper (draft)

docs pending.
ENH: More flexible GLM.

Eat regressors form datasets, the mapper itself, auto-add constants.
BF: Do not modified input values.
RF: GLMMapper

To abstract away from NiPyGLMMapper.
NF+RF: GLM Mapper with statsmodels + plus convergence test.
BF: Missed argument rename
Merge pull request #189 from hanke/event_glm_model

NF: 1st-level HRF-modeling for eventrelated_dataset() (fixes gh-188)