Decrease logo image size and increase title length

Author: mwaskom

README.md

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-PySurfer
-========
+PySurfer: Neuroimaging visualization in Python
+==============================================
 
-![](doc/logo_files/pysurfer_logo_small_crop.png)
+<img src=doc/logo_files/pysurfer_logo_small_crop.png width=500px style="horizontal-align:middle">
 
 PySurfer is a Python package for interacting with a cortical surface
 representations of neuroimaging data. It extends Mayavi's powerful