export assayData<-, fData<-, featureNames<- in NAMESPACE

Export overloaded methods in NAMESPACE to avoid the following warnings of
`R CMD check` for packages depending on MSnbase:

    no visible global function definition for ‘fData<-’
add pData<- to pSet manual page
Merge pull request #40 from lgatto/visibleFunctionDefinition

`R CMD check` warning: "no visible global function definition"