Changes in version 0.1.2

- Lots of improvements to the documentation and examples.

- Added the generalized Stuval index with the index_stuval() function.

- Added the weights_update() function for the common case of price-updating weights in an arithmetc index.

- Functions that say the input should be a vector now check if the input is a vector. This prevents non-name attributes from causing problems in the calculations (like dim).

- weights_change() now allows 'M' to be NA/NaN, returning a vector of NAs.

- Error messages are now more informative.
