v0.5.1

Yields v0.5.1

Diff since v0.5.0

Closed issues:

• provide function for efficient vector of accumulation/discount (#23)
• document conversion functionality in readme (#29)
• Inconsistent return type (#40)

Merged pull requests:

• call first value from rates, not the function rate (#35) (@alecloudenback)
• we want element-wise division when time is a vector instead of matrix division (#41) (@alecloudenback)