Update toFixed to round negative values away from zero and handle exponential notation

[gustavlrsn]

Fixes an inconsistency between toFixed and formatNumber referenced in #164.

Updating toFixed so that it rounds negative values away from zero, to be in consistence with how formatNumber and thus formatMoney has been working.

Also adding some tests to formatNumber to clarify this current (& past) behaviour of rounding negative values away from zero.

Before:

console.log(accounting.toFixed(-61.005, 2)); // "-61.00"
console.log(accounting.formatNumber(-61.005, 2)); // "-61.01"

with this fix:

console.log(accounting.toFixed(-61.005, 2)); // "-61.01"
console.log(accounting.formatNumber(-61.005, 2)); // "-61.01"

[gustavlrsn]

Updated the method to handle values that are represented in exponential notation, and fixing #166. Also refactored the method a little bit with more comments explaining what it does. Also added tests for testing incoming values in exponential/scientific notation.