BootstrapProgressBar should match the Bootstrap Specification

[jamie-beardedhen]

http://getbootstrap.com/components/#progress

BootstrapProgressBar should support:

• An arbitrary text label centered in the drawn progress
• Multiple stacked progress of different colors
• XML Attributes and mutators for the minimum and maximum progress values (integers, defaulting to 0 & 100).
• Throwing an exception whenever the progress is set to a value outside the min/max range (or when altering the min/max.