Add currency conversion command and some other fixes

**The Issue**
The bot includes no method to convert currencies. This command adds this functionality. Additionally, I noticed aliases were not functional during development and fixed the issue.

**Justification for this PR:**
In IRC channels with multiple users from different locations, currency conversion can be useful.

**PR Breakdown**
* Adds a rate lookup interface an an implementation.
* Adds unit tests for the Converter class.
* Adds some util methods to the StringUtil class and appropriate tests.

**IRC Log**
````irc
<@lol768> \cur GBP EUR 5
<Nexus> 5.00 GBP is 6.10 EUR to the nearest penny.
<@lol768> \cur GBP USD 5
<Nexus> 5.00 GBP is 8.49 USD to the nearest penny.
<@lol768> \cur USD GBP 25
<Nexus> 25.00 USD is 14.72 GBP to the nearest penny.
````