Merge pull request #4 from timholy/master

Improved finite differencing
strcat() => string()
Merge branch 'master' of github.com:johnmyleswhite/Calculus.jl
remove require("Calculus.jl")

no longer needed, "using" is enough for installed packages.
Merge pull request #5 from rened/patch-1

remove require("Calculus.jl")
Fixed symbolic calculations for new Expr() call
Add mutating finite differencer
Add an abstract symbolic variable type, methods for that, and Miles
Lubin's chainRule code.
Remove operator overloading and add @sexpr based on input from Miles.
More differentiation rules for chainRule.
Use `@sexpr` to generate chainRule methods from a table.
Simplified simplify().
Function renames and file reorganization.
Complete file re-org.
More derivative rules plus a simplify tweak.
Fix bug that Miles caught and fix tests.
Merge pull request #7 from tshort/master

[for discussion] an abstract symbolic type and chain rule code
Add travis configuration
Merge pull request #9 from aviks/travis

Add travis configuration
Minor code cleanup