BAM open fails when index file is fname.bai instead of fname.bam.bai

What steps will reproduce the problem?
1. Have test.bam and test.bai 
2. open test.bam .
3. IOError raised since test.bam.bai doesn't exist

What is the expected output? What do you see instead?
0.7.5 checks for the existence of either fname.bai or fname.bam.bai and on 
success tries to open fname.bam.bai which causes IOError since fname.bam.bai is 
non-existent. Earlier versions only checked for fname.bam.bai and on success 
opened fname.bam.bai which was fine. Either 0.7.5 should not check for 
fname.bai or actually load index from fname.bai if it exists. 

What version of the product are you using? On what operating system?
0.7.5 on Linux 

Please provide any additional information below.

Original issue reported on code.google.com by margho...@binatechnologies.com on 27 Nov 2013 at 1:27

Merged into: #116

lixin6135 / pysam

BAM open fails when index file is fname.bai instead of fname.bam.bai #145